Solved NPE which I can't seem to identify.

Hello everyone, I'm developing a plugin and when I register an event, it's been throwing an NPE. Generally, this means that one of the arguments I provided is null. (Please correct if wrong.) I've verified both arguments to registerEvents();, and they are both defined correctly (but apparently that's not the case since it's throwing an NPE. )

I don't know what to do. I'm hoping one of you can help me out on this one.

Server version: 1.13.2
Spigot library version: 1.13.2

Stacktrace:

Code:

Error occurred while enabling NNClasses v0.0.1 (Is it up to date?)
java.lang.NullPointerException: null
at org.bukkit.plugin.SimplePluginManager.registerEvents(SimplePluginManager.java:520) ~[craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at io.github.tsjiptsjip.classplugin.listeners.InvClickListener.<init>(InvClickListener.java:21) ~[?:?]
at io.github.tsjiptsjip.classplugin.Main.onEnable(Main.java:14) ~[?:?]
at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:254) ~[craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at org.bukkit.plugin.java.JavaPluginLoader.enablePlugin(JavaPluginLoader.java:331) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at org.bukkit.plugin.SimplePluginManager.enablePlugin(SimplePluginManager.java:402) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at org.bukkit.craftbukkit.v1_13_R2.CraftServer.enablePlugin(CraftServer.java:421) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at org.bukkit.craftbukkit.v1_13_R2.CraftServer.enablePlugins(CraftServer.java:347) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at net.minecraft.server.v1_13_R2.MinecraftServer.l(MinecraftServer.java:575) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at net.minecraft.server.v1_13_R2.MinecraftServer.a(MinecraftServer.java:537) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at net.minecraft.server.v1_13_R2.MinecraftServer.a(MinecraftServer.java:415) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at net.minecraft.server.v1_13_R2.DedicatedServer.init(DedicatedServer.java:270) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at net.minecraft.server.v1_13_R2.MinecraftServer.run(MinecraftServer.java:680) [craftbukkit-1.13.2.jar:git-Bukkit-282dad1]
at java.lang.Thread.run(Thread.java:748) [?:1.8.0_181]